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ATOMIC ABSORPTION SPECTROPHOTOMETRY COOKBOOK

Section 1

Basic Condit ions of Analysis of Atomic

Absorpt ion Spectrophotometry

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Atomic Absorption Spectrophotometry Cookbook

Section 1

CONTENTS

1. Principal of Atomic Absorption Spectrophotometry .................................... 1

1.1 Why atoms absorb light .......................................................................................... 1

1.2 Relation between light absorption rate and atomic density .................................... 2

1.3 Sample atomization method ................................................................................... 3

a) Flame atomic absorption .................................................................................... 3

b) Electro-thermal atomic absorption .................................................................... 4

2. Basic Condition for Analysis .............................................................................. 9

2.1 Conditions of equipment ........................................................................................ 9

a) Analysis line ...................................................................................................... 9

b) Slit width ............................................................................................................ 13

c) Lamp current value ............................................................................................ 14

2.2 Analysis conditions of flame atomic absorption .................................................... 15

a) Flame selection .................................................................................................. 15

b) Mixing ratio of oxidant and fuel gas .................................................................. 17

c) Beam position in flame ...................................................................................... 17

2.3 Analysis conditions of electro-thermal atomic absorption ..................................... 18

a) Drying condition ................................................................................................ 18

b) Ashing condition ................................................................................................ 19

c) Atomizing condition .......................................................................................... 21

d) Sample injection quantity .................................................................................. 23

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1. Principal of Atomic Absorption Spectrophotometry

1.1 Why atoms absorb light

The atomic absorption spectrometry uses absorption of light of intrinsic wavelengths by atoms.

All atoms are classified into those having low energies and those having high energies. The

state having low energies is called the ground state and the state having high energies is called theexcited state.

The atom in the ground state absorbs external energies and is put in the excited state. For

example, sodium is mainly in two excited states, having higher energies by 2.2eV and 3.6eV

respectively than in the ground state, as shown in Fig. 1.1. (eV is a unit to measure energies and is

called an electron volt.) When 2.2eV energy is given to the sodium atom in the ground state, it

moves up to the excited state in (I) and when 3.6eV energy is given, it moves up to the excited

state in (II).

Energy is given as light, and 2.2eV and 3.6eV respectively correspond to energy of light at

589.9nm and 330.3nm wavelength.

In the case of sodium in the ground state, only light of these wavelengths are absorbed and no

other wavelength light is absorbed at all.

Fig. 1.1 Sodium energy states

The difference between energies in the ground state, and in the excited state is fixed by the

element and wavelength of light to be absorbed. Atomic absorption spectrometry uses the hollow

cathode lamp (HCL).

The HCL gives off light characteristic to the elemental wavelength being measured. Thus, the

light absorbed measures the atomic density.

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1.2 Relation between light absorption rate and atomic density

When light of certain intensity is given to many atoms in the ground state, part of this light is

absorbed by atoms.

The absorption rate is determined by the atomic density.

Fig. 1.2 Principle of atomic absorption

When light of Io intensity is given to density C, atoms speed in length 1 as shown in Fig. 1.2.

The light is absorbed and its intensity is weakened to I.

The following formula is formed between I and Io.

I = Io ek l c

(k: Proportional constant)

or log I = k l c

This is called the Lambert-Beer's Law, and -log I value is absorbance.

The above formula indicates that absorbance is proportional to atomic density.

When absorbance is measured on samples of 1, 2 and 3 ppm for example and plotted, a

straight line is obtained as shown in Fig. 1.3. Absorbance and concentration represented

graphically is called the calibration curve.

When the absorbance of an unknown sample is obtained, the concentration can be determined

from the graph as shown.

Io

Io

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Concentration (ppm)

Concentration of unknown

sample

Fig. 1.3 Calibration curve

1.3 Sample atomization method

The principle mentioned above can be applied to light absorption of Free atoms. A Free

atom means an atom not combined with other atoms. However, elements in the sample to be

analyzed are not in the free state, and are combined with other elements invariably to make a so-

called molecule. For example, sodium in sea water mainly combines with chlorine to form a NaCl

(Sodium chloride) molecule. Absorption cannot be done on samples in the molecule state,

because molecules do not absorb light.

The combination must be cut off by some means to free the atoms. This is called atomization.

The most popular method of atomization is dissociation by heat - samples are heated to a high

temperature so that molecules are converted into free atoms. This method is classified into the

flame method, in which a chemical flame is used as the heat source; and a flameless method, in

which a very small electric furnace is used.

a) Flame atomic absorption

The flame is produced by a burner for atomization and this is the most popular method. It is

standard in almost all atomic absorption devices available on the market at present.

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Fig. 1.4 Flame atomic absorption

A typical diagram of the burner is shown in Fig. 1.4.

This figure explains measurement of calcium contained in the sample liquid as calcium

chloride. The sample is atomized by a nebulizer at first. Then, big water drops are discharged

to the drain, and only a fine mist is mixed with fuel, and oxidant in the atomizer chamber and

sent to the flame.

When they get in the flame, the mist evaporates instantaneously and fine particles of

calcium chloride molecules are produced. When these particles further advance in the flame,

calcium chloride is dissolved by heat and free calcium atoms and chloride atoms are produced.If a beam of light at wavelength 422.7nm(Ca) is introduced through this part of the flame,

atomic absorption can be measured. In the upper part of the flame, some of calcium atoms are

combined with oxygen to become calcium oxide and some are further ionized. Therefore,

atomic absorption does not show sufficient sensitivity even if light is given to such a position.

Many combinations of various gases have been tested as the flame for atomization. In

consideration of analysis sensitivity, safety, easy use, cost and other points; there are four

standard flames used: air-acetylene, nitrous oxide-acetylene, air-hydrogen and argon-

hydrogen. These flames are used for each element depending on the temperature and gas

characteristics.

b) Electro-thermal atomic absorption

The atomization method using a flame is still popularly used as the standard atomization

method due to good reproducibility of measured values and easy use. However, a major defect

of the flame method is the atomization rate out of all sample quantity used is about 1/10 and

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the remaining 9/10 is discharged to the drain. Therefore, it has been pointed out that

atomization efficiency is low and analysis sensitivity is not so high.

Electro-thermal atomic absorption (flameless method), using a graphite tube, improves the

above defects to elevate sensitivity 10 to 200 times as much. This method was originated by

Dr. L'vov of Russia.

Fig. 1.5 Flameless atomizer

In the electro-thermal atomic absorption method, the sample is injected in the formed

graphite tube and an electric current of 300 ampere(maximum) is applied to the tube. The

graphite is heated to a high temperature and the elements in the sample are atomized.

If light from the light source is sent through the tube, light is absorbed when they areatomized.

In an actual measurement, after the sample is injected in the tube, heating is done in three

stages as shown in Fig. 1.6. That is, in the drying stage, the tube is heated to about 100oC and

water in the sample evaporates completely. Then, in the ashing stage, the tube is heated to

400oC to 1000

oC and organic matter and other coexistent matter dissolve and evaporate.

Lastly, in the atomizing stage, it is heated to 1400oC to 3000

oC and metallic salts left in the

tube are atomized. Heating is usually done by changing the temperature in steps shown by the

solid line in Fig. 1.6 (step heating). Depending on the sample, when the decomposition

temperature of coexistent matter is close to its atomization temperature, heating is done by

changing temperature continuously (ramp mode heating).

Heating must be done under the conditions (temperature, heating time, and temperature

raising method), which suit the type of element and composition of the sample to be measured.

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If heating is started after the optimum conditions are set on the equipment in advance, the

tube is automatically heated according to the set temperature program.

Fig. 1.6 Heating program and absorption curve accord ing to

electro-thermal atomic absorption

c) Other atomic absorption methods

Methods having higher sensitivity than normal flame atomic absorption or electro-thermal

atomic absorption are often used for special elements including arsenic, selenium and mercury.

They use chemical reactions in the process of atomization to vaporize in the form of an atom

or simple molecule.

Hydride vapor generation technique

The hydride vapor generation technique is used to make the sample react on sodium

borohydride.

It is acidified with HCl to reduce the object metal, and combine it with the hydrogen in

order to produce a gaseous metal hydride. This gas is sent to the high temperature

atomization unit for measurement.

As, Se, Sb, Sn, Te, Bi, Hg and other metals produce a metal hydride by this method.

Fig. 1.7 shows the block diagram of the hydride generating equipment. The peristalsistic

pump is used to send the sample, 5M hydrochloric acid and 0.5% sodium borohydride

solution to the reaction coil. The metal hydride is generated in the reaction coil and the gas-

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liquid separator is used to separate the gas phase and liquid phase. Argon gas is used as the

carrier gas. The gas phase is sent to the absorption cell, which is heated by the air-acetylene

flame, and the metallic element is atomized.

Peristaltic

pump

Fig. 1.7 Block diagram of hydraulic generating equipment

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Reduction vapor atomization

Mercury in solution is a positive ion. When it is reduced to a neutral ion, it vaporizes as

a free atom of mercury, at room temperature. Tin (II) chloride is used as a reducing agent

and mercury atoms are sent to the atomic absorption equipment with air as the carrier gas.

Fig. 1.8 shows the block diagram of the mercury analysis equipment. 200ml of thesample is put in the reaction vessel, and tin (II) chloride is added for reduction. When air is

sent to the gas flow cell through the drying tube, atomic absorption by mercury is measured.

Fig. 1.8 Block diagram of mercury analysis equipment

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2. Basic Condition for Analysis

The equipment must be set at the optimum analysis conditions to obtain the best measurement

results.

Optimum conditions generally vary with the element and with the composition of the sample,

even if the same elements are contained. Therefore, it is necessary to fully study the measuringconditions in actual analysis.

2.1 Conditions of equipment

a) Analysis line

Light from the hollow cathode lamp shows a number of primary and secondary spectrums

of cathode elements and filler gas. They are complicated particularly with 4, 5, 6, 7 and 8

families in the middle of the periodic table, showing several thousand spectrums.

Parts of many spectral lines contribute to atomic absorption. The atomic absorption analysis

selects and uses the spectral line of the biggest atomic absorbance.

The spectral line having absorption sensitivity suitable for the analysis may be used. This

depends on the concentration range where the elements in the sample are measured.

An element may have two or more spectral lines showing atomic absorption as in Table 2.1.

It is desirable to check absorption sensitivity and emission intensity of these spectral lines.

Also study the concentration range in which each wavelength is measured in order to avoid the

dilution error when the concentration is high as in the main component analysis.

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Table 2.1 Analysis lines and absorption sensitivit ies

(Characteristics of hollow cathode lamp and handling method

Hamamatsu Photonics)

Ele-

ment

Analysis line

wavelength (nm)

Absorption

sensitivity Flame type

Ag 328.07

338.29

10

5.3

Air-C2H2

Al 309.27

396.15

237.13

237.30

10

8.6

2.0

N2O-C2H2

As 193.70

197.20

189.00

10

6.2

5.0

Ar-H2

Au 242.80

267.59

10

5.5

Air-C2H2

B 249.68

249.77

208.89

10

8.2

N2O-C2H2

Ba 553.55

350.11

10

0.01

N2O-C2H2

Be 234.86 10 N2O-C2H2

Bi 223.06

222.83

306.77

10

3.0

2.5

Air-C2H2

Ca 422.67

239.86

10

0.05

Air-C2H2

Cd 228.80

326.11

10

0.02

Air-C2H2

Co 240.73

251.98

243.58

346.58

10

4.4

1.3

0.5

Air-C2H2

Cr 357.87

425.44

427.88

428.97

10

4.4

2.7

1.0

Air-C2H2

Ele-

ment

Analysis line

wavelength (nm)

Absorption

sensitivity Flame type

Cs 852.11 10 Air-C2H2

Cu 324.75

327.40

217.89

218.17

222.57

10

4.7

1.2

1.0

0.6

Air-C2H2

Dy 404.59

421.17

418.68

10

8.9

8.0

N2O-C2H2

Er 400.79

415.11

386.28

10

5.9

5.5

N2O-C2H2

Eu 459.40

462.72

466.19

10

8.7

7

N2O-C2H2

Fe 248.33

271.90

371.99

385.99

10

2.7

0.9

0.6

Air-C2H2

Ga 294.36

287.42

403.30

10

8.2

4.2

Air-C2H2

Gd 407.89

422.59

378.31

10

10

10

N2O-C2H2

Ge 265.16

270.96

269.13

10

4.8

3.0

N2O-C2H2

Hf 286.64

307.29

289.83

10

9.3

5.0

N2O-C2H2

Hg 253.65 10 Reductionvaporization

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Ele-

ment

Analysis line

wavelength (nm)

Absorption

sensitivityFlame type

Ho 410.38

416.30

10

5.8

N2O-C2H2

In 303.94

325.61

410.48

10

9.4

4.0

Air-C2H2

Ir 208.88

266.47

284.97

10

2.6

1.5

Air-C2H2

K 766.49

769.90

404.41

10

2.5

0.03

Air-C2H2

La 550.13

403.72

357.44

364.95

10

2.3

0.8

0.5

N2O-C2H2

Li 670.78

323.26

10

0.06

Air-C2H2

Lu 331.21

328.17

10

7.1

N2O-C2H2

Mg 285.21

202.58

10

0.9

Air-C2H2

Mn 279.48

280.11

403.08

10

4.7

1.1

Air-C2H2

Mo 313.26

319.40

320.88

10

4.7

0.8

Air-C2H2

Na 589.00

589.59

330.23

330.30

10

4.8

0.02

Air-C2H2

Nb 334.91

405.89

10

8

N2O-C2H2

Ele-

ment

Analysis line

wavelength (nm)

Absorption


Nd 492.45

463.42

10

0.8

N2O-C2H2

Ni 232.00

341.48

352.45

231.10

351.50

10

5.1

5.0

2.0

0.9

Air-C2H2

Os 290.90

305.86

263.71

330.16

10

4.5

4.0

2.0

N2O-C2H2

Pb 217.00

283.33

261.41

202.20

10

3.9

0.2

0.1

Air-C2H2

Pd 244.79

247.64

276.31

340.46

10

6.8

2.2

1.5

Air-C2H2

Pr 495.13

513.34

504.55

10

6.9

2.5

N2O-C2H2

Pt 265.95

292.98

10

2.0

Air-C2H2

Rb 780.02

794.76

10

4.6

Air-C2H2

Re 346.05

346.47

345.19

10

5.3

3.5

N2O-C2H2

Rh 343.49

339.69

328.09

10

2.8

0.2

Air-C2H2

Ru 349.89 10 Air-C2H2

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Ele-

ment

Analysis line

wavelength (nm)

Absorption


Sb 217.58

206.83

231.15

212.74

10

7.0

3.6

1.0

Air-C2H2

Sc 391.18

390.74

402.37

402.04

326.99

10

7.6

7.0

5.0

3.0

N2O-C2H2

Se 196.03

203.99

10

2.0

Ar-H2

Si 251.61

250.69

251.43

252.41

288.16

10

3.0

3.0

2.5

0.7

N2O-C2H2

Sm 429.67

484.17

10

8.2

N2O-C2H2

Sn 224.61

286.33

233.48

10

6.2

6.0

Air-C2H2

Sr 460.73

407.77

10

0.6

Air-C2H2

Ta 271.47

264.75

275.83

10

5.9

2.6

N2O-C2H2

Tb 432.64

431.88

390.14

10

8.5

6.0

N2O-C2H2

Ele-

ment

Analysis line

wavelength (nm)

Absorption


Te 214.27

225.90

10

1.0

Air-C2H2

Ti 364.27

365.35

398.98

10

9.0

4.0

N2O-C2H2

Tl 276.78

377.57

10

4.2

Air-C2H2

Tm 371.79

410.58

374.41

10

6.5

6

N2O-C2H2

V 318.40

306.64

305.63

10

3.8

3.0

N2O-C2H2

W 255.14

400.87407.44

10

3.60.1

N2O-C2H2

Y 410.23

412.83

407.74

10

8.5

8

N2O-C2H2

Yb 398.79

346.43

246.45

10

3.2

2.0

N2O-C2H2

Zn 213.86

307.59

10

0.002

Air-C2H2

Zr 360.12 10 N2O-C2H2

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b) Slit width

Concerning spectral lines emitted from the hollow cathode lamp, their wavelength is an

independent line or complicated nearby line depending on the element.

Calcium and magnesium have no other spectral lines near the object analysis line as shown

in Fig. 2.1.In case of such analysis lines, slit width is set considerably greater to obtain sufficient

energy.

Fig. 2.1 Lamp spectrums

Nickel has many spectral lines near the object analysis line of 232.0nm (2320A). Because

light of these nearby wavelengths is hardly absorbed with nickel atoms, the resolving power

spectroscope must be increased (slit width is narrowed) to separate only 232.0nm light.

If measurement is made in the low resolving power condition, the measurement sensitivity

grows worse and at the same time, linearity of the calibration curve becomes deteriorated. (Fig.

2.2)

Cobalt (Co), iron (Fe), manganese (Mn) and silicon (Si) show complicated spectrums like

nickel.

The resolving power of the spectroscope must be below 2A to measure these elements

accurately.

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Ni Concentration

Fig. 2.2 Slit width and calibration curve

c) Lamp current value

If the hollow cathode lamp operating conditions are notproper, the spectral line causes a

Doppler broadening or broadening due to self-absorption, to affect the measured value.

Doppler broadening is caused by the temperature of the hollow cathode lamp space, which

does not contribute to lamp emission. As the hollow cathode lamp current increases,

luminance increases; thus the spectral lines broaden causing absorption sensitivity to drop as

shown in Fig. 2.3.

The life of the hollow cathode lamp is generally indicated by ampere-hour (A.Hr).

Therefore, the life is shortened if the current value is increased.

Such being the case, a low cathode lamp lighting current value is desirable but luminance

drops if it is too low. Detector sensitivity must be increased, but noise results from it.

The lamp current value is determined by three factors: luminance (noise) of the above lamp,

absorption sensitivity, and lamp life.

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Fig. 2.3 Sensit ivity by changing the hollow cathode lamp current value

2.2 Analysis condi tions of flame atomic absorption

a) Flame selection

Air-acetylene, air-hydrogen, argon-hydrogen, and nitrous oxide-acetylene are the standard

types of flames used in atomic absorption analysis.

These flames vary in temperature, reducibility and transmission characteristics. The

optimum flame must be selected according to the element being analyzed, and properties of

the sample.

Air-acetylene flame (AIR-C2H2)This flame is most popularly used and about 30 elements can be analyzed by this.

Nitrous oxide-acetylene flame (N2O-C2H2)

This flame has the highest temperature among flames used for atomic absorption.

Aluminum, vanadium, titanium, etc. combine strongly with oxygen in the air-acetylene

flame and other relatively low temperature flames. Free atoms decrease and make

measurement difficult. However, such elements are hard to combine with oxygen due to

high temperature in the nitrous oxide-acetylene flame making satisfactory measurement

possible.

The nitrous oxide-acetylene flame can also be substituted for the elements analyzed by

the air-acetylene flame. The high temperature of the nitrous oxide-acetylene flame has very

small interferences.

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Air-hydrogen flame (Air-H2) and argon-hydrogen flame (Ar-H2)

The hydrogen flame absorbs very little light from the cathode lamp, only in the short

wavelength region. (Refer to Fig. 2.4).

Therefore, measurement can be done with a smaller background noise, in this short

wavelength region, than with the air-acetylene flame. Those wavelength elements are As,Se, Zn, Pb, Cd, Sn, etc.

Since the argon-hydrogen flame absorbs the smallest amount of light from 200nm and

below, it is typically used.

The disadvantage of using a hydrogen type flame is that it is susceptible to interferences

due to its low temperature.

Fig. 2.4 Light absorbance of various flames

Table 2.2 shows the maximum temperature of each flame.

Table 2.3 shows elements and types of flames used.

Table 2.2 Flame temperature

Flame typeMaximum

temperature

Argon-hydrogen

Air-hydrogen

Air-acetylene

Nitrous oxide-acetylene

1577oC

2045oC

2300oC

2955oC

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Table 2.3 Elements and flames used for measurement

V Cr Mn Fe Co

78

Ni29

79

Cu

80

Zn

5

13

31

Ga

82

Ge As

1a 2a 3b 4b 5b 6b 7b 8 1b 2b 3a 4a 5a 6a 7a 01 2

3 4 6 7 8 9 10

11

19

37

55

87

12

20

38

56

88

21

39

57

89

22

40

72

23

41

73

24

42

74

25

43

75

26

44

76

27

45

77

28

46 47

30

48 49

81

14

32

50

15

33

51

83

16

34

52

84

17

35

53

85

18

36

54

86

Fr Ra Ac

H

Li

Na

K

Rb

Cs

Be

Mg

Ca

Sr

Ba

Sc

Y

La

Ti

Zr

Hf

Nb

Ta

Mo

W

Tc

Re

Ru

Os

Rh

Ir

Pd

Pt

Ag

Au

Cd

Hg

B

Al

In

Tl

C

Si

Sn

Pb

N

P

Sb

Bi

O

S

Se

Te

Po

F

Cl

Br

I

At

He

Ne

Ar

Kr

Xe

Rn***

58

90 91

60

92

61

93

62

94

63

95

64

96

65

97

66

98

67

99

68

100

69

101

70

102

71

103

Ce Nd Pm Sm Eu Gd Dy Ho Er Tm Yb Lu59

Pr Tb

ThAcril-nides Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

***Hg is analyzed by the cold vapor mercury technique.

;

;

Lantha-

nides

N2O-C2H2Flame

AIR-C2H2Flame

Flames generally used for atomikc absorption analysis. Elements which are not colored cannot be analysis.

AIR-H2Flame

Ar-H2Flame

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

b) Mixing ratio of oxidant and fuel gas

The mixing ratio of oxidant and fuel gas is one of the most important items among

measurement conditions of atomic absorption analysis. The mixing ratio affects flame

temperature and environment, and determines generating conditions of ground state atoms.

Therefore, the flame type as well as the beam position in the flame described in the next

paragraph, control 80 to 90 percent of absorption sensitivity and stability (reproducibility).

Cu, Ca, Mg, etc. increase sensitivity in the oxidizing flame containing more oxidant (fuel lean

flame) and Sn, Cr, Mo, etc. increase sensitivity in the reducing flame containing more fuel gas

(fuel rich flame).

Because extremely fuel lean or fuel rich may cause instability, it must be set at the optimum

value depending on the target object. Absorption values by changing the acetylene flow are

measured with constant air flow and the condition showing the maximum absorption value is

obtained. Because the above study is concerned with the burner position described in the next

paragraph, acetylene flow and burner height are adjusted to decide the optimum mixing ratio.

c) Beam position in flame

Distribution of ground state atoms generated in the flame are not uniform depending on the

element, but varies depending on the flame mixing ratio. Fig. 2.5 shows distribution of ground

state atoms when the gas mixing ratio is changed in the measurement of chromium. It indicates

that atom distribution and density change when the mixing ratio is changed. Because

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absorption sensitivity changes with the beam position in the flame, the burner position is set so

that the beam passes the optimum position.

Fig. 2.5 Distribut ion of chromium atoms in air-acetylene flame

(Atomic absorption spectroscopy, W, salvin)

2.3 Analysis condi tions of electro-thermal (flameless) atomic absorption

Electro-thermal (flameless) atomic absorption conducts heating in three basic stages for

sample atomization.

The first step is the Drying Stage, which evaporates the solvent.

The second step is the Ashing Stage; to dissolve organic matter in the sample and evaporate

the salts.The third step is the Atomization Stage. If needed, a Cleaning Stage can be set. The following

describes each condition setting.

a) Drying condition

This stage is to evaporate the solvent. The heating temperature and time are set depending

on the type and quantity of the solvent used for measurement.

The standard heating temperature for evaporating the solvent is 60oC to 150

oC for water-

type samples, or 50oC to 100

oC for organic-type samples.

The heating time is based on 1 second per 1l of the sample. The heating temperature and

time are set so that the solvent is evaporated completely. If the drying condition is not perfect,

a fizzle (bumping) is heard or smoke blows through the graphite tube hole when the next stage

is entered. To clearly examine, set the measurement mode to the deuterium lamp mode, and

check if the absorption peak is exactly zero. The above is the judgment criteria.

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There are two heating methods: Step and Ramp modes. In the step mode, the furnace is

directly heated to the target temperature, at the beginning of the stage, and maintained at a

constant temperature until the end of the stage. In the ramp mode, heating is performed at a

constant rate so that the target temperature is reached by the end of the stage. The sample

injected in the graphite tube diffuses(spreads) in the tube. If too much sample is injected orsample viscosity is high, the sample may stay on the surface of the graphite tube.

If sharp heating is done, the sample bubbles or bumps. When bubbling or bumping occurs,

the sample flies off from the filler port and diffuses at random in the tube, making

reproducibility worse.

In such a case, it is effective to make heating by step mode at a slightly lower temperature

than the solvent evaporating temperature. However, ramp mode heating is easier to set the

condition. Ramp mode heating and step mode heating may be combined to increase the drying

efficiency.

The pyrolytic graphite tube has small filtration due to its fine surface. Therefore, special

care is necessary. Spreading conditions of the sample into the tube varies with the graphite

tube temperature and sample injection to worsen reproducibility. So, it is desirable to inject the

sample under the constant temperature of 10 to 15oC higher than room temperature.

b) Ashing condition

If organic matter, or salts, exist in the atomization stage, background absorption (chemical

interference) occurs giving an error in the analysis value.

Therefore, organic matter and salts are evaporated in the ashing stage where possible.

It is desirable to increase the ashing temperature as high as possible to remove organic

matter and salts.

However, if the ashing temperature is increased, evaporation of the target metal happens

and errors in the analysis values occur. Therefore, it must have a limit. The volatilization

(evaporation) temperature of the target metal is checked in advance to decide the ashing

temperature.

Fig. 2.6 shows the relation between the ashing temperature and absorption sensitivity of a

lead solution with nitric acid. The ashing temperature and absorption sensitivity every 100oC

suggest that volatilization occurs from 500oC in the case of lead.

The condition is studied on lead nitrate, but the volatilizing temperature must be checked on

the same chemical species as the sample to be measured. That is because the volatilizing

temperature varies with the chemical species of the target metal generated in the ashing stage.

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One means to decrease background absorption is to dilute the sample, but it cannot be

applied when density of the target metal is very low. A matrix modifier is used in such a case.

Palladium (II) nitrate and nickel nitrate are used as the matrix modifier. They have the effect of

increasing the volatilizing temperature of the target metal as mentioned in 5.3. That is, becausethe ashing temperature can be raised, background absorption can be decreased and absorption

sensitivity can be increased.

Step mode heating and ramp mode heating are available as the heating method in the same

way as drying. In step mode heating, salts in the graphite tube may blow out from the sample

filler port after completion of drying. Generally, the method combined with ramp mode

heating and step mode heating; ramp heating is done from drying temperature to ashing

temperature, taking 10 to 20 seconds and then the ashing temperature is kept for the specified

time.

Heating time in the ashing stage varies with the quantity of salt, or organic matter contained

in the sample, andis generally 30 to 60 seconds. Whether ashing is perfect or not for this

heating time can be checked by magnitude of background absorption. The deuterium lamp

mode is set as the measuring mode and absorption peak in the atomizing stage is measured.

The time when absorption magnitude does not change, even if the ashing time is extended, is

the setting time.

c) Atomizing condition

This step is to atomize the target metal. Heating may be made for about 5 seconds at a

slightly higher temperature than the atomizing temperature of the target metal. Absorption

sensitivity, when the atomizing temperature is changed, is checked to decide the atomizing

temperature. Fig. 12.8 shows the relation between the atomizing temperature and absorption

sensitivity. It indicates that heating may be done at 2500oC or above.

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Fig. 2.8 Relation between aluminum atomizing temperature and sensitivi ty

If the atomizing temperature is set too high for metals of low melting points includingcadmium and lead, the atom staying time in the tube becomes extremely short and sensitivity

may drop. Metals including boron, molybdenum and calcium are easily maintained in the

graphite tube. Therefore, atomization is done at a temperature as high as possible or pyrolytic

graphite tube is used.

About 1l/min of argon is run through the graphite tube in the drying and ashing stages. If

argon gas is run in the atomizing stage, sensitivity drops sharply.

Therefore, argon is stopped. Sensitivity can be adjusted five times as much by changing

argon flow from 0 to 1.5l/min to adjust absorption sensitivity.

Step heating is generally used. When background absorption at the atomization stage is big,

atomic absorption, background absorption, and measurement should be made by ramp heating.

The heating time is set so that the atomic absorption peak returns to 0 level within the

heating time. However, when the metal is easy to stay in the graphite tube or background

absorption is big and does not return to 0 level, the time when the peak returns to the specified

level is set as heating time, and cleaning is done thereafter.

Cleaning is done to evaporate metal and salt, which remains in the graphite tube, at the end

of the atomizing stage. Heating can be done sufficiently for 2 to 3 seconds at the maximum

temperature of 3000oC but lower temperature is desirable where possible.

The standard cleaning temperature is the atomization temperature plus 200oC. Cleaning is

done at about 2500oC for cadmium and lead, which have low atomization temperatures.

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d) Sample injection quantity

Proportional relations do not work between the sample quantity injected in the graphite tube

and absorption sensitivity. This is because the diffusion area in the tube and filtration depth

vary with sample injection quantity. Therefore, the calibration line can be prepared by

changing the injection quantity of the standard solution from the specified density.Solutions of different densities are injected in the specified quantity at one time. The

injection quantity of the standard sample is naturally the same as that

of the sample.

The maximum sample injection quantity is 50l but diffusion and filtering depth vary with

a difference in physical properties of the sample. It spreads to the low temperature part, or

overflows to the filler port often dropping analysis accuracy. Therefore, 10 to 20l is ideal.

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CookBook 01